Background Preterm birth is a major cause of perinatal morbidity and mortality worldwide. In many countries, the preterm birth rate in women with a multiple pregnancy is increasing, mostly due to an increase in iatrogenic preterm birth. Aims To investigate trends in preterm birth in twin pregnancies in Victoria, Australia, in relation to maternal and perinatal complications. Materials and methods We conducted a retrospective population‐based cohort study in all women with a twin pregnancy who delivered at or after 20 weeks of gestation in the state of Victoria, Australia between 2007 and 2017. Annual spontaneous and iatrogenic preterm birth rates were calculated and trends analysed. Incidence of adverse pregnancy outcomes, maternal complications and risk factors for preterm birth were analysed. Results We studied 12 757 women with a twin pregnancy. Between 2007 and 2017 the preterm birth rate increased from 641/1231 (52%) to 803/1158 (69%), mainly due to an increase in iatrogenic preterm birth from 342/1231 (28%) to 567/1158 (49%). This was irrespective of the presence of pregnancy complications. Our study showed neither a decrease in perinatal mortality from 28 weeks of gestation nor in preterm average weekly prospective stillbirth risk. Conclusion Preterm birth rates in twins in Victoria are increasing, mainly driven by an increase in iatrogenic preterm birth. This occurred both in complicated and non‐complicated twin pregnancies, and has not been accompanied by reduction in perinatal mortality from 28 weeks.